Accommodation, the process by which eyes focus on near objects, occurs in humans through the controlled contraction of the ciliary muscle coupled with controlled elastic relaxation of the crystalline lens. The latter event results in a change in lens shape which alters its refractive properties. The means by which the two processes are coupled, however, is a matter of controversy, and neither of the currently accepted theories can adequately explain the age-related erosion of accommodative amplitude (presbyopia). The purpose of this competing continuation proposal is to characterize changes in human crystalline lens properties as a function of increasing lens age, both in situ with non-invasive techniques and in vitro using isolated post-mortem lenses, and to consider accommodation and accommodative loss in terms of these studies. In vivo studies on selected human subjects as a function of age will include standard clinical techniques such as pachymetry and keratometry, and the use of the Scheimpflug method to collect slit-lamp photographs of the accommodating lens for each subject at 2 diopter intervals over the entire accommodative range; when analyzed, these photos will supply information on relative locations of the cornea, anterior lens surface, and posterior lens surface, as well as characterization of all discernible curvatures associated with the lens (e.g., boundaries between adjacent zones of discontinuity) as functions of accommodative state. In vitro studies will employ both standard and innovative methods to characterize lens elasticity, physical and biodhemical properties, and optical properties of isolated post-mortem lenses as a function of age and sex. Computer-based statistical packages will be used to correlate these results as a function of age, sex, and (where applicable) visual acuity, and a revised computer based model on accommodation, which incorporates these data and which is applicable to all ages, will be formulated. These studies are designed to fill gaps in the current understanding of the human lens aging process, and to consider directly the changes in the human lens which may be associated with the development of presbyopia. Some of the parameters which can be studied non-invasively, as well as those that can be studied invasively, are directly correlated to subject age and can thus be used as "biomarkers" for the aging process.